The invention is directed to a process for the production of spherical fuel or absorber elements from high temperature reactors consisting of a graphitic matrix and embedded therein coated particles by molding a mixture of graphite molding powder containing resin binder with coated nuclear fuel or absorber particles to form a sphere (or balls), carbonizing the binder resin in a furnace with flushing gas and calcining in a vacuum at up to about 2000.degree. C.
Spherical fuel or absorber elements for high temperature reactors, such as are described for example in German patent No. 1909871, contain the nuclear fuel or absorber material in the form of coated particles which are embedded in a graphitic matrix. In the production of this matrix material finely ground highly crystalline types of natural and/or electrographite are treated with a resin binder and further processed by mixing or kneading processes as well as further grinding to the required granulation to form a so-called graphite molding powder. Subsequently the coated particles, in a given case after an additional encasing step, in which they are coated with a molding powder layer of the order of magnitude of 100 .mu.m, are coated with graphite molding powder and molded to spherical fuel elements. The resin binder is then carbonized in a carbonizing step, subsequently a purification and degassing of the elements is carried out whereby these elements are calcined at temperatures above 1600.degree. C. in a vacuum.
As resin binder for the matrix material there can be used pitches as well as thermoplastic and thermosetting synthetic resins, especially there can be employed thermoplastic and thermosetting synthetic resins based on phenol, e.g. phenolformaldehyde and phenol-furfural.
In carbonizing this resin binder there are formed organic products of cracking. Furthermore there takes place a temporary softening of the binder, especially when using thermoplastic synthetic resins or pitches, during the temperature increase at the beginning of the carbonization process. During this phase the form stability of the fuel or absorber element molding is reduced.
These problems which customarily occur in the carbonization of resin binders and liberate even more highly condensed, difficultly volatile cracked products and temporary reduced form stability of the molding have lead to the carrying out of the carbonization of the spherical nearly exclusively batchwise in pot furnaces. Thereby the spheres are positioned on charging sheets in such manner that they do not contact each other in order to safely prevent a possible adhesion with products of carcking.
Thereby either the storage of the moldings must be carried out in such manner that no impermissible shaping occurs during the softening phase of the binder, or there must be employed thermosetting resin binders to maintain the form stability. Alternatively the moldings can also be carbonized in a push through furnace. Hereby the spheres must be transported through the furnace individually in suitable boats in order to prevent any possible mutual contact and adherence or an impermissible deformation. In all cases the products of cracking are removed from the carbonizing furnace by so-called flushing gas.
Because of the considerable expense in the loading and unloading of the charging position or the boats both processes are cumbersome and only permit a limited throughput per furnace unit.
Therefore it was the problem of the present invention to develop a process for the production of spherical fuel or absorber elements for high temperature reactors consisting of (or consisting essentially of) a graphitic matrix and particles embedded therein by molding a mixture of graphite molding powder containing a binder resin with coated nuclear fuel or absorber particles to form a sphere, carbonizing the resin binder in a furnace with gas flushing and vacuum calcining up to about 2000.degree. C., whereby a continuous method of operation with high throughputs can be produced, without adherence of the spheres or without taking into consideration a temporary form of instability and without employing transportation aids.